Light-emitting device containing flattened anisotropic colloidal semiconductor nanocrystals and processes for manufacturing such devices

ABSTRACT

A device that emits light in response to an electrical or optical excitation, such as LEDs, displays, e-readers, device includes at least one anisotropic flat colloidal semiconductor nanocrystal whose smallest dimension, namely the thickness, is smaller than the other two by a factor of at least 1.5, the emitted light having an intensity and a polarization that vary according to the angle formed by the light emitting direction and the normal to the largest surface of the flat nanocrystal. The device allows to realize a light-emitting device exhibiting simultaneously a high emission spectral finesse and allows proper control of the wavelength, the directivity and/or polarization of the emitted light, and thus increases the brightness and color gamut of displays composed of such a device. Such devices are found for example in displays, televisions, mobile phones, tablets, or computers. The various embodiments of these devices are also presented.

TECHNICAL FIELD

This invention relates to devices that emit light in response to anelectrical or luminous excitation, such as light emitting diodes,displays, e-readers, etc. Such devices are found for example indisplays, televisions, cell phones, tablets or computers. The devices ofthe type in question still have, despite steady progress, a certainnumber of limitations both in regard to color rendering and thecontrast, brightness, energy efficiency, and the visual impressiondepending on the angle of observation.

BACKGROUND

To restore the colors in all their variety, one proceeds in general byadditive synthesis of at least three complementary colors, especiallyred, green and blue. In a chromaticity diagram, the subset of availablecolors obtained by mixing different proportions of these three colors isformed by the triangle formed by the three coordinates associated withthe three colors red green and blue. This subset constitutes what iscalled a gamut.

The majority of color display devices operates on this three-colorprinciple: each pixel consists of three sub-pixels, one red, one greenand one blue, whose mixture with different intensities can reproduce acolorful impression.

A luminescent or backlit display such as a computer LCD screen has topresent the widest possible gamut for an accurate color reproduction.For this, the composing sub-pixels must be of the most saturated colorspossible in order to describe the widest possible gamut. A light sourcehas a saturated color if it is close to a monochromatic color. From aspectral point of view, this means that the light emitted by the sourceis comprised of a single narrow band of wavelengths. We recall that ahighly saturated shade has a vivid, intense color while a less saturatedshade appears more bland and gray.

It is therefore important to have light sources whose emission spectraare narrow and therefore of saturated colors.

For example, in the case of a color display, the red, green and bluesub-pixels composing it must have a spectrum maximizing the gamut of thedisplay system, which amounts to exhibiting the narrowest possibleemission from a spectral point of view.

It is possible to distinguish two types of polychromic light-emittingdisplays:

-   -   Backlit displays, in which a white light coming from the        backlighting is filtered by color filters and whose intensity is        controlled by a liquid crystal system: these are the        liquid-crystal displays (LCD),    -   Directly emissive displays, in which each pixel consists of at        least three sub-pixels corresponding to three basic colors. Each        sub-pixel is a light emitter independently addressed, often        through a matrix or multiplexed system, which emitted light        intensity is then directly set. This is the case of plasma        screens and light-emitting diodes screens such as OLEDs screens        (for “Organic Light Emitting Diode”). These devices use a        material emitting light in response to an excitation.

In LCD screens, the color of the pixels is determined by the filteringof a white primary source by red, green and blue filters. The spectra ofthe three sub-pixels therefore correspond to the multiplication of theemission spectrum of the primary source, which is usually an array ofwhite LEDs or a cold cathode fluorescent tube, by the transmissionspectrum of the filters used. The fact of optimizing the spectra of theprimary light source or of the color filters therefore allows improvingthe gamut. However, most of the light emitted by the white primarysource is either reabsorbed by the polarizers and color filters thatmake up the screen, or deflected by diffusion and waveguide effect inthe different layers. Thus it does not reach the observer, whichseverely limits the energy efficiency of liquid-crystal displays. Ittherefore requires, to limit power consumption, to seek agamut—brightness compromise.

To increase the color gamut and brightness of the screen withoutsignificant change of the filters and the primary light source, it hasbeen recently proposed to add a fluorescent film containing colloidalquantum dots between the light source and the pixels in order to modifythe spectrum of the light coming from the source after passing throughthe film in question and thus to enhance the saturation of the threesub-pixels^(4, 5). However, this solution, even though improving thegamut, decreases the brightness of the screen.

It was also proposed to replace the filters by green, red and bluewavelength converters which absorb the primary light, blue orultraviolet for instance, and which retransmit the specific color ofeach converter. For this, a material containing fluorophores whichabsorb the light from a primary excitation source and re-emit it at ahigher wavelength is used. However, this solution has issues ofstability, fluorescence efficiency, and spectral finesse of thefluorophores used in said wavelength converters.

The directly emissive displays, such as displays composed oflight-emitting diodes, are potentially lower in energy consumption;there is little or no loss by filtering. However, when usingsemi-conductor layers, such as in inorganic diodes or polymer layers asin the case of OLEDs, light losses by total internal reflections in saidlayers, reduce the total light that reaches the observer.

In directly emissive displays, the nature of the excitation can bevarious:

-   -   Electric, by charges injection as in the case of organic or        inorganic light-emitting diodes.    -   Optical, by absorption of photons of wavelength shorter than the        emission wavelength, as in the case of wavelength converters or        plasma screens.

Many emissive materials have been proposed to try to cover the entirevisible spectrum. Thus, organic fluorophores, present for example inOLEDs have a high quantum yield in the visible, commonly greater than90%. They are generally poorly stable, degrading for example due tooxidation or radiations, which reduces the lifetime of the containingdevices. Moreover the width of the fluorescence spectra can be quitelarge, which does not allow to obtain a large gamut. Finally, theoptimal excitation wavelength can be different for each fluorophore,making their integration into a system with a common excitation sourcedifficult.

Oxides or complexes of the rare earths are emissive materials commonlyused, such as in plasma screens and OLEDs. In this case, the emissivematerial is much more stable as it is weakly sensitive to oxidation. Thewidth of the emission peaks can be very small, of the order of tennanometers, but the absorption cross section of these materials is low,which may require the use of large quantities. Moreover their emissionwavelength is not tunable, because it is defined by the material, forexample the rare earth complex used. This is an important limitation,which does not allow this type of transmitters to cover the entirevisible spectrum.

The emissive materials of plasma screens or OLEDs sometimes include atransition metal oxide. As for the rare earth oxides, the fluorescentmaterial is very stable as it is weakly sensitive to oxidation. However,the fluorescence spectral width is very high, typically from fifty toseveral hundred nanometers, which does not allow to generate saturatedcolors and thus to present a high gamut.

Semiconductor nanoparticles, commonly called “quantum dots”, are analternative as emissive material. Said objects have a narrowfluorescence spectrum, approximately 30 nm full width at half maximum,and offer the possibility to emit in the entire visible spectrum as wellas in the infrared with a single excitation source in theultraviolet^(8, 9). However, they do not allow to optimize the lightreceived by the observer and thus the energetic efficiency of thedevice. In this case an improvement of the gamut of polychromic displaysrequires a finesse of the emission spectra that is not accessible forquantum dots.

The object of the present invention is thus to provide a newlight-emitting device allowing a great spectral emission finesse, aperfect control of the emission wavelength, the directivity and/orpolarization of the emitted light. The present invention thussignificantly improves the brightness and color gamut of displayscomposed of said devices.

SUMMARY

The present invention relates to a component, for emitting light inresponse to the activation of excitation means, comprising a support andat least one anisotropic flat semiconductor colloidal nanocrystal whosesmallest dimension, namely the thickness, is smaller than the other twodimensions by a factor of at least 1.5, the normal to the largestsurface of the at least one anisotropic flat semiconductor colloidalnanocrystal being substantially parallel or substantially perpendicularto the support; said emitted light having an intensity and apolarization that vary according to the angle formed by the lightemitting direction and the normal to the largest surface of the flatnanocrystal.

In one embodiment, the at least one anisotropic flat semiconductorcolloidal nanocrystal is a colloidal semiconductor nanosheet.

In one embodiment, the at least one anisotropic flat colloidalsemiconductor nanocrystal comprises at least one of the compounds ofgroup IV, group III-V, group II-VI, Group III-VI, Group I-III-VI, groupII-V or group IV-VI, or mixture thereof.

In one embodiment, the at least one anisotropic flat colloidalsemiconductor nanocrystal comprises at least one of the followingcompounds: Si, Ge, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe,PbS, PbSe, PbTe, CuInS₂, CuInSe₂, AgInS₂, AgInSe₂, CuS, Cu₂S, Ag₂S,Ag₂Se, Ag₂Te, InN, InP, InAs, InSb, In₂S₃, Cd₃P₂, Zn₃P₂, Cd₃As₂, Zn₃As₂,ZnO, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb FeS₂, TiO₂, Bi₂S₃,Bi₂Se₃, Bi₂Te₃.

In one embodiment, the at least one anisotropic flat colloidalsemiconductor nanocrystal is a heterostructure comprising an anisotropicflat colloidal semiconductor nanocrystal

In one embodiment, the at least one anisotropic flat colloidalsemiconductor nanocrystal is totally covered by a semiconductor ofdifferent chemical composition.

In one embodiment, the at least one anisotropic flat colloidalsemiconductor nanocrystal has a narrow fluorescence spectrum, with afull width at half maximum of less than 30 nm, or 25 nm, or 23 nm, orpreferably 20 nm.

In one embodiment, the support is transparent to the emitted light in atleast one direction to an observer situated outside the component, thesupport surface through which the light is emitted towards the observerbeing defined as emitting face.

In one embodiment, the support comprises a liquid having the propertiesof liquid crystals.

In one embodiment, the support is flexible or rigid.

In one embodiment, the support comprises an inorganic material or anorganic material.

In one embodiment, the support comprises a polymer material.

In one embodiment, the component comprises at least two anisotropic flatcolloidal semiconductor nanocrystals having different characteristics,dimensions and/or chemical compositions and/or emission wavelengths.

In one embodiment, substantially all of the flat nanocrystals of thecomponent have the normal to their surfaces substantially parallel to agiven direction.

The present invention also relates to a light emitting system with atleast one excitation means comprising means for applying to theanisotropic flat colloidal semiconductor nanocrystal an electromagneticfield, including a light source, where at least part of the emittedlight is absorbed by the anisotropic flat colloidal semiconductornanocrystal, such as a gallium nitride diode.

The present invention also relates to an apparatus comprising at leastone component and/or at least one system according to the presentinvention.

The present invention also relates to a component to emit light inresponse to the activation of excitation means, comprising a support anda plurality of anisotropic flat colloidal semiconductor nanocrystalswhose smallest dimension, namely the thickness, is smaller than theother two dimensions by a factor of at least 1.5, wherein at least 50%of the plurality of nanocrystals have their normal to their largestsurface substantially parallel to a given direction.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals have their normal to the largest surfacesubstantially parallel or substantially perpendicular to the support.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals is a plurality of anisotropic flat colloidalsemiconductor nanosheets.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals comprises at least one compound of group IV,group III-V, group II-VI, group III-VI, group I-III-VI, group II-V orgroup IV-VI, or mixture thereof.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals comprises at least one of the followingcompounds: Si, Ge, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe,PbS, PbSe, PbTe, CuInS₂, CuInSe₂, AgInS₂, AgInSe₂, CuS, Cu₂S, Ag₂S,Ag₂Se, Ag₂Te, InN, InP, InAs, InSb, In₂S₃, Cd₃P₂, Zn₃P₂, Cd₃As₂, Zn₃As₂,ZnO, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb FeS₂, TiO₂, Bi₂S₃,Bi₂Se₃, Bi₂Te₃.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals comprises at least one heterostructurecomprising an anisotropic flat colloidal semiconductor nanocrystal.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals comprises at least one anisotropic flatcolloidal semiconductor nanocrystal totally covered with a semiconductorof different chemical composition.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals exhibits a narrow fluorescence spectrum, witha full width at half maximum less than 100 nm, 75 nm, 50 nm, 40 nm, 30nm, 25 nm, 24 nm, 23 nm, 22 nm, 21 nm or 20 nm.

In one embodiment, the support is transparent for the emitted light inat least one direction towards an observer located outside thecomponent, the support surface through which the light is emittedtowards the observer being defined as emitting face.

In one embodiment, the support comprises a liquid having liquid crystalsproperties.

In one embodiment, the support is flexible or rigid.

In one embodiment, the support comprises an inorganic material or anorganic material.

In one embodiment, the support comprises a polymer material.

In one embodiment, the plurality of anisotropic flat colloidalsemiconductor nanocrystals comprises at least two anisotropic flatcolloidal semiconductor nanocrystals having different characteristics,dimensions and/or chemical compositions and/or emission wavelengths.

In one embodiment, the emitted light has an intensity and a polarizationwhich vary according to the angle formed by the light emitting directionand the normal to the largest surface of the plurality of anisotropicflat colloidal semiconductor nanocrystals.

The present invention also relates to a light emitting system comprisinga component according to the present invention and at least oneexcitation means comprising means for applying to the plurality ofanisotropic flat colloidal semiconductor nanocrystals an electromagneticfield, in particular a light source, wherein at least a portion of theemitted light is absorbed by the plurality of anisotropic flat colloidalsemiconductor nanocrystals; such as a gallium nitride diode.

The present invention also relates to an apparatus comprising acomponent or a system according to the present invention.

DESCRIPTION OF THE INVENTION

The present invention relates to a device that emits light in responseto the activation of excitation means, said device comprising at leastone anisotropic flat colloidal semiconductor nanocrystal whose smallestdimension, namely the thickness, is smaller than the other two by afactor of at least 1.5, said emitted light having a polarization and anintensity that vary according to the angle formed by the light emittingdirection and the normal to the largest surface of the flat nanocrystal.

In a preferred embodiment, the light emitting device comprises at leastone assembly (i.e. a plurality) of anisotropic flat colloidalsemiconductor nanocrystals.

In the following description, nanocrystal refers, except otherwisespecified, to an anisotropic flat colloidal semiconductor nanocrystal,preferably a nanosheet. In the following description, the normal to thenanocrystal is defined as the normal to the largest surface of theanisotropic flat colloidal semiconductor nanocrystal. Within the presentdescription, nanoplatelet, nanosheet, and nanoplate have the samemeaning

The present invention also relates to an excitation device comprisingthe excitation means for said light-emitting device. The excitationmeans comprising means of injection of electric charges, electronsand/or holes in the anisotropic flat colloidal semiconductor nanocrystalor means of applying to the anisotropic flat colloidal semiconductornanocrystal an electromagnetic field.

An anisotropic flat colloidal semiconductor nanocrystal, according tothe invention, is a crystalline particle which at least one of thedimensions, preferably the thickness, is smaller than the other two by afactor of at least 1.5.

In one embodiment of the invention, the crystalline particle asdescribed above is a nanosheet or a heterostructure containing a quantumdot or a nanosheet.

In one embodiment of the invention, a heterostructure is a core/shelltype structure in which the shell totally or partially covers the core.

In one embodiment, the heterostructure, as described above, comprises acore composed of a material and a shell composed of a material differentfrom the material of the core.

In one embodiment of the invention, the shell can be composed of severalidentical or different materials.

In one embodiment, the heterostructure as described above comprises acore comprising a quantum dot or a nanosheet and a shell, saidheterostructure having at least one of its dimensions, preferably thethickness, smaller than the other two by a factor of at least 1.5.

In one embodiment, the excitation means comprise means of injectingelectrical charges, electrons and/or holes in the anisotropic flatcolloidal semiconductor nanocrystal.

In one embodiment, the excitation means comprise means of applying tothe anisotropic flat colloidal semiconductor nanocrystal anelectromagnetic field.

In one embodiment, the electromagnetic field source is a light sourcewhich at least part of the emitted light is absorbed by the anisotropicflat colloidal semiconductor nanocrystal.

In one embodiment, the light source which at least part of the emittedlight is absorbed by the anisotropic flat colloidal semiconductornanocrystal is a gallium nitride (GaN) based diode.

In one embodiment, the excitation means comprise at least one electricor magnetic dipole coupled by electromagnetic interactions to theanisotropic flat colloidal semiconductor nanocrystal.

In one embodiment, for a single anisotropic flat colloidal semiconductornanocrystal the ratio of intensity of the electromagnetic field betweenthe normal to the largest surface of the nanocrystal and along the planeof largest surface of the nanocrystal is higher than 0.50, 0.55, 0.60;0.65, 0.70, 0.75, 0.80, 0.85, 0.90 or 0.95.

In each of said direction, light exhibits polarization. In oneembodiment, said polarization is linear, circular or a combinationthereof.

In one embodiment, the polarization along the normal of the anisotropicflat colloidal semiconductor nanocrystal is circular. In one embodiment,the polarization along the surface of the anisotropic flat semiconductorcolloidal nanocrystal is linear.

In one embodiment, an assembly of anisotropic flat semiconductorcolloidal nanocrystals having separately a contrast of polarizationexhibits a macroscopic polarization of its emission.

In one embodiment, the macroscopic emission polarization of the assemblyof anisotropic flat semiconductor colloidal nanocrystals results fromthe global alignment of the nanocrystals. The contrast of polarizationis highly related to the ratio of nanocrystals presenting the sameorientation.

In one embodiment, the polarization contrast of the oriented assembly ofanisotropic flat semiconductor colloidal nanocrystals is higher than20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%,90%, or 95%.

The contrast of polarization C is defined as follow:

${C = \frac{I_{\bot} - I_{\parallel}}{I_{\bot} + I_{\parallel}}};$

where I is the intensity of light in a given direction or polarization.

In one embodiment, more than 50% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 55% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 60% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 65% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 70% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 75% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 80% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 85% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 90% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In one embodiment, more than 95% of the anisotropic flat colloidalsemiconductor nanocrystals of the assembly have the same orientation,meaning having the same normal direction to the plane of the sheetwithin +/−40°, +/−30°, +/−20°, +/−10° or +/−5°.

In a preferred embodiment, said orientation is defined as substantiallyparallel or substantially perpendicular to the normal of the emittingface of the component.

In one embodiment, the nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure has at least one of itsdimensions, preferably the thickness, of 0.3 nm to less than 1 μm, 0.3nm to less than 500 nm, 0.3 nm to less than 250 nm, 0.3 nm to less than100 nm, 0.3 nm to less than 50 nm, 0.3 nm to less than 25 nm, 0.3 nm toless than 20 nm, 0.3 nm to less than 15 nm, 0.3 nm to less than 10 nm,0.3 nm to less than 5 nm.

In one embodiment, the nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure has at least one lateraldimension of 0.3 nm to at least 1 mm, 0.3 nm to 100 μm, 0.3 nm to 10 μm,0.3 nm to 1 μm, 0.3 nm to 500 nm, 0.3 nm to 250 nm, 0.3 nm to 100 nm,0.3 nm to 50 nm, 0.3 nm to 25 nm.

In one embodiment, at least one of the lateral dimensions (length orwidth), preferably both lateral dimensions, of the anisotropic flatcolloidal semiconductor nanocrystal represent(s) at least 1.5 times itsthickness, at least 2 times its thickness, at least 2.5 times itsthickness, at least 3 times its thickness, at least 3.5 times itsthickness, at least 4 times its thickness, at least 4.5 times itsthickness, at least 5 times its thickness.

In one embodiment, the thickness of the anisotropic flat colloidalsemiconductor nanocrystal is 0.5 nm to less than 1 μm, 0.5 nm to lessthan 500 nm, 0.5 nm to less than 250 nm, 0.5 nm to less than 100 nm, 0.5nm to less than 50 nm, 0.5 nm to less than 25 nm, 0.5 nm to less than 20nm, 0.5 nm to less than 15 nm, 0.5 nm to less than 10 nm, 0.5 nm to lessthan 5 nm.

In one embodiment, the lateral dimensions of the anisotropic flatcolloidal semiconductor nanocrystal are at least 0.75 nm to at least 1.5mm.

In one embodiment, at least one of the lateral dimensions of theanisotropic flat colloidal semiconductor nanocrystal is 2 nm to 1.5 mm,2 nm to 1 mm, 2 nm to 100 μm, 2 nm to 10 μm, 2 nm to 1 μm, 2 nm to 100nm, 2 nm to 10 nm.

In one embodiment, the nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure comprises semiconductors ofgroup IV, III-V, II-VI, II-V, III-VI or/and group IV, group IIIA-VA,group IIA-VIA, group IIIA-VIA, group IA-IIIA-VIA, group IIA-VA, groupIVA-VIA, group VIB-VIA, group VB-VIA, or group IVB-VIA.

In one embodiment, the nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure, comprises a material of thecomposition M_(x)E_(y) where:

M is Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Ge, Pb, Sb, Sn, Pd, Fe, Au, Ti,Bi, W, Mo, V or a mixture thereof,

E is O, S, Se, Te, N, P, As or a mixture thereof, and

x and y are independently a decimal number of 0 to 5, and notsimultaneously zero.

In one embodiment, the nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure, comprises a material of thecomposition M_(x)N_(y)E_(z), wherein:

-   -   M is selected from Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Sn, Ge,        Pb, Sb, Sn, Pd, Fe, Au, Ti, Bi, W, Mo, V or a mixture thereof;    -   N is selected from Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Sn, Ge,        Pb, Sb, Sn, Pd, Fe, Au, Ti, Bi, W, Mo, V or a mixture thereof;    -   E is selected from O, S, Se, Te, N, P, As or a mixture thereof;        and    -   x, y and z are independently a decimal number from 0 to 5, at        the condition that when x is 0, y and z are not 0, when y is 0,        x and z are not 0 and when z is 0, x and y are not 0.

According to one embodiment, he nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure is made of a quaternarycompound such as InAlGaAs, ZnAgInSe or GaInAsSb.

In one embodiment, the nanosheet, or the initial nanosheet ornanocrystal present in the heterostructure comprises at least one of thefollowing: Si, Ge, Sn, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe,HgTe, PbS, PbSe, PbTe, CuInS₂, CuInSe₂, AgInS₂, AgInSe₂, CuS, Cu₂S,Ag₂S, Ag₂Se, Ag₂Te, InN, InP, InAs, InSb, In₂S₃, Cd₃P₂, Zn₃P₂, Cd₃As₂,Zn₃As₂, ZnO, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, FeS₂, TiO₂,Bi₂S₃, Bi₂Se₃, Bi₂Te₃, MoS₂, WS₂, VO₂, and alloys and mixtures thereof.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal comprises semiconductors of group IV, III-V, II-VI,I-III-VI, II-V, III-VI.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal or the assembly of anisotropic flat colloidal semiconductornanocrystals comprises semiconductors of group IV, group IIIA-VA, groupIIA-VIA, group IIIA-VIA, group IA-IIIA-VIA, group IIA-VA, group IVA-VIA,group VIB-VIA, group VB-VIA, or group IVB-VIA.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal comprises a material of the composition M_(x)E_(y) where:

M is Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Ge, Pb, Sb or a mixturethereof,

E is O, S, Se, Te, N, P, As or a mixture thereof, and

x and y are independently a decimal number of 0 to 5, and notsimultaneously zero.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal or the assembly of anisotropic flat colloidal semiconductornanocrystals comprises a material of the composition M_(x)E_(y) where:

M is Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Ge, Pb, Sb, Sn, Pd, Fe, Au, Ti,Bi, W, Mo, V or a mixture thereof,

E is O, S, Se, Te, N, P, As or a mixture thereof, and

x and y are independently a decimal number of 0 to 5, and notsimultaneously zero.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal or the assembly of anisotropic flat colloidal semiconductornanocrystals comprises a material of the composition M_(x)N_(y)E_(z),wherein:

-   -   M is selected from Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Sn, Ge,        Pb, Sb, Sn, Pd, Fe, Au, Ti, Bi, W, Mo, V or a mixture thereof;    -   N is selected from Zn, Cd, Hg, Cu, Ag, Al, Ga, In, Si, Sn, Ge,        Pb, Sb, Sn, Pd, Fe, Au, Ti, Bi, W, Mo, V or a mixture thereof;    -   E is selected from O, S, Se, Te, N, P, As or a mixture thereof;        and    -   x, y and z are independently a decimal number from 0 to 5, at        the condition that when x is 0, y and z are not 0, when y is 0,        x and z are not 0 and when z is 0, x and y are not 0.

According to one embodiment, the anisotropic flat colloidalsemiconductor nanocrystal or the assembly of anisotropic flat colloidalsemiconductor nanocrystals is made of a quaternary compound such asInAlGaAs, ZnAgInSe or GaInAsSb.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal or the assembly of anisotropic flat colloidal semiconductornanocrystals comprises at least one of the following: Si, Ge, CdS, CdSe,CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, PbS, PbSe, PbTe, CuInS₂,CuInSe₂, AgInS₂, AgInSe₂, CuS, Cu₂S, Ag₂S, Ag₂Se, Ag₂Te, InN, InP, InAs,InSb, In₂S₃, Cd₃P₂, Zn₃P₂, Cd₃As₂, Zn₃As₂, ZnO, AlN, AlP, AlAs, AlSb,GaN, GaP, GaAs, GaSb, FeS₂, TiO₂, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, MoS₂, WS₂, VO₂or a mixture thereof.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal or the assembly of at anisotropic flat colloidalsemiconductor nanocrystal comprises an alloy of the materials listedabove.

In one embodiment the anisotropic flat colloidal semiconductornanocrystal comprises a nanocrystal or an initial nanosheet partially ortotally covered on at least one side by at least a monolayer or a layerof an additional material.

In one embodiment where multiple monolayers or layers cover all or partof the nanocrystal or initial nanosheet, these single layers or layerscan comprise the same material or different materials.

For the purposes of the present invention, the term “layer” refers to afilm or a continuous or partial layer being at least one atom thick. Theterm “monolayer” refers to a film or a continuous or partial layer beingone atom thick. The atoms constituting the layer or the monolayer may bethe same or different.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal is homostructured, i.e. the initial nanocrystal or nanosheetand the at least one monolayer or layer are made of the same material.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal is heterostructured, i.e. the initial nanocrystal ornanosheet and the at least one monolayer or layer are composed of atleast two different materials.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal can comprise the initial nanocrystal or nanosheet and 1, 2,3, 4, 5 or more monolayers or layers covering all or part of the initialnanocrystal or nanosheet, said monolayers or layers being of identicalcomposition to the initial nanocrystal or nanosheet or being ofdifferent composition than that of the initial nanocrystal or nanosheetor being of different composition there between.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal can comprise the initial nanocrystal or nanosheet and atleast 2, 3, 4, 5 or more monolayers or layers, wherein the firstmonolayer or layer deposited covers all or part of the initialnanocrystal or nanosheet and the at least second monolayer or layerdeposited covers all or part of the previously deposited monolayer.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal has a core/shell structure, i.e. an initial nanocrystal ornanosheet of given composition covered over its entire surface by atleast one monolayer or layer of different composition than that of theinitial nanocrystal or nanosheet.

Thus, the material obtained is composed of a stack of films of which atleast one is of identical chemical composition or different chemicalcomposition than that of the initial nanocrystal or nanosheet, thesurface of each film covering completely the surface of the film onwhich it is deposited.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal comprises a de-wetted structure, i.e. an initial nanocrystalor nanosheet of a given composition partly covered by at least onemonolayer or layer of the same composition or different composition thanthat of the initial nanocrystal or nanosheet.

Thus, the material obtained is composed of a stack of films of which atleast one is of identical chemical composition or different chemicalcomposition than that of the initial nanocrystal or nanosheet, thesurface of each film partially covering the surface of the film on whichit is deposited.

Thus, in one embodiment, the anisotropic flat colloidal semiconductornanocrystal has a heterostructure comprising the materials listed above.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal is doped with a lanthanide or a transition metal.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal is covered by a semiconductor of different chemicalcomposition.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal is covered by organic molecules.

In one embodiment, the organic molecules are chosen between thiols,amines, carboxylic acids, phosphonic acids, phosphinic acids,phosphines.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal is a colloidal semiconductor nanosheet.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal displays a narrow fluorescence spectrum, of full width athalf maximum less than 100 nm, than 75 nm, than 50 nm, than 40 nm, than30 nm, than 25 nm, than 24 nm, than 23 nm, than 22 nm, than 21 nm orthan 20 nm.

In one embodiment, the anisotropic flat colloidal semiconductornanocrystal emits light at a wavelength between 350 and 1500 nm.

The present invention also relates to a component comprising at leastone light-emitting device in response to the activation of excitationmeans, according to an embodiment of the invention, and a support forsaid at least one device, said support being transparent to the lightemitted by the device in at least one direction towards an observersituated outside the component, the support surface through which lightis emitted towards the observer being defined as emitting face.

Thus, for the purpose of the present invention, a component is definedas the combination of at least one light-emitting device in response tothe activation of excitation means as described in the present inventionand a support in which is placed said at least one device.

In one embodiment, the at least one light-emitting device in response tothe activation of excitation means is in contact with the support.

In one embodiment, the at least one light-emitting device in response tothe activation of excitation means is integrated in the support.

In one embodiment, the at least one light-emitting device in response tothe activation of excitation means is in contact with a first supportand covered by a second support; said first and second supports being ofthe same nature or of different nature.

The present invention also relates to a light-emitting system includinga component as described in the present invention and at least oneexcitation device according to an embodiment of the invention whereinthe excitation means of the at least one device are integrated into thesupport.

In one embodiment, the excitation means of the at least one device areintegrated into the support.

In one embodiment, the excitation means of the at least one device areexternal to the support.

In one embodiment, the component comprises at least two devices sharingthe same excitation means.

In one embodiment, the support comprises a liquid.

In one embodiment, the liquid has properties of liquid crystals.

In one embodiment, the support comprises at least one organic material.

In one embodiment, the support comprises at least one organicsemiconducting material.

In one embodiment, the support comprises a polymer material.

In one embodiment, the support is rigid.

In one embodiment, the support is flexible. In one embodiment, thepolymer is a polyelectrolyte.

In one embodiment, the polymer comprises chemical functions capable ofsubstituting themselves to the surface ligands of the nanocrystals.

In one embodiment, the polymer is a poly(methyl methacrylate), apolystyrene, a polycarbonate, a polyethylene, a polyethyleneterephthalate, an epoxide, a polyester, a polysiloxane.

In one embodiment, the polymer comprises a semiconducting polymer.

In one embodiment, the polymer comprises polythiophene, P3HT, MDMO PPV,MEH-PPV, PEDOT, PEDOT:PSS, PCBM, PCNEPV, polyfluorene, PSS.

In one embodiment, the support comprises at least one inorganicmaterial.

In one embodiment, the inorganic material is a semiconductor.

In one embodiment, the semiconductor comprises at least one of thefollowing: CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, AlN, GaN, InN, AlP, GaP,InP, AlAs, GaAs, InAs, C, Si, Ge.

In one embodiment, the preferred anisotropic flat colloidalsemiconductor nanocrystal is a CdSe core with a CdZnS shell.

In one embodiment, the semiconductor is doped, it contains in minoramounts an element producing an excess or a default of electronscompared to the sole semiconductor.

In one embodiment, the inorganic material is a glass such as silica.

In one embodiment, the inorganic material comprises an oxide chosenfrom: TiO₂, ITO (indium oxide doped with tin), NiO, ZnO, SnO₂, SiO₂,ZrO₂, FTO (tin oxide doped with fluorine).

In one embodiment, the inorganic material is a metal.

In one embodiment, the component comprises at least two devices, the atleast two anisotropic flat colloidal semiconductor nanocrystals of theat least two devices having different characteristics. In a preferredembodiment, the component comprises at least two devices, the at leasttwo assemblies of anisotropic flat colloidal semiconductor nanocrystalsof the at least two devices having different characteristics.

In one embodiment, said characteristics are the dimensions.

In one embodiment, said characteristics are the chemical compositions.

In one embodiment, said characteristics are the emission wavelengths.

In one embodiment, the component is characterized in that it comprisesat least two flat nanocrystals, the normals to the surfaces of the atleast two flat nanocrystals being substantially parallel to a givendirection.

In one embodiment, the component is characterized in that it comprisesat least two flat nanocrystals, the normals to the surfaces of the atleast two flat nanocrystals being substantially perpendicular to a givendirection.

In one embodiment, substantially all of the flat nanocrystals of thecomponent have their normal to their surfaces substantially parallel toa given direction.

In one embodiment, substantially all of the flat nanocrystals of thecomponent have their normal to their surfaces substantiallyperpendicular to a given direction

In one embodiment, the normal to the surface of the at least one flatnanocrystal of the component is substantially parallel to the normal ofthe emitting face.

In one embodiment, the normal to the surface of the at least one flatnanocrystal of the component is substantially perpendicular to thenormal of the emitting face.

In one embodiment, the normals to the surface of more than 50%, 55%,60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of flat nanocrystals of theassembly of the component is substantially parallel to the normal of theemitting face.

In one embodiment, the normals to the surface of more than 50%, 55%,60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of flat nanocrystals of theassembly of the component is substantially perpendicular to the normalof the emitting face.

In one embodiment, the support of the component comprises means toorient the at least one flat nanocrystal according to at least onepreferred direction. In one embodiment, the support of the componentcomprises means to orient the at least one assembly of flat nanocrystalsaccording to at least one preferred direction.

In one embodiment, the component is made by a process wherein the flatnanocrystals are deposited on a support surface with the normal to theirsurface substantially parallel to the normal to the support surface.

In one embodiment, the component is made by a process wherein more than50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of flat nanocrystalsof the assembly are deposited on a support surface with the normal totheir surface substantially parallel to the normal to the supportsurface.

In one embodiment, the component is made by a process wherein the flatnanocrystals are deposited on a support surface with the normal to theirsurface substantially perpendicular to the normal to the supportsurface.

In one embodiment, the component is made by a process wherein more than50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of flat nanocrystalsof the assembly are deposited on a support surface with the normal totheir surface substantially perpendicular to the normal to the supportsurface.

In one embodiment, the component is made by a process wherein thesupport surface comprises a texturing.

In one embodiment, the component is made by a process comprising a slowevaporation phase of a solution containing the flat nanocrystals.

In one embodiment, the slow evaporation phase is realized while applyingan electric field.

In one embodiment, the component is made by a process comprising a stepof depositing the flat nanocrystals by spin coating a solutioncontaining the flat nanocrystals.

In one embodiment, the component is made by a process comprising a stepof depositing the flat nanocrystals by dip coating of the substrate in asolution containing the flat nanocrystals.

In one embodiment, the solution containing the flat nanocrystalscomprises also an organic compound such as a polymer or a monomer.

In one embodiment, the component is made by a process comprising a stepof layer-by-layer electrostatic adsorption of the flat nanocrystalshaving a surface charge.

In one embodiment, the component is made by a process comprising thetransfer of a film of oriented flat nanocrystals previously formed.

In one embodiment, the film of nanocrystals is obtained by slowevaporation on a liquid surface of a solution containing thenanocrystals.

The present invention also relates to a method for manufacturing thecomponent according to an embodiment of the invention in which the flatnanocrystals are deposited on a support surface with the normal to theirsurface substantially parallel to the normal to the surface support.

Within the purpose of the present invention substantially parallel orsubstantially perpendicular means that the angle formed by the normalsto the two surfaces is +40° to −40°, or +30° to −30°, or +20° to −20°,or +10° to −10°, or +5° to −5°.

The present invention also relates to an apparatus comprising at leastone component and/or a system wherein means are provided to activateeach component and/or system by excitation means, independent oneanother.

In one embodiment, the activation means of the apparatus are of electrictype and comprise at least two electrodes arrays in a matrixarrangement.

In one embodiment, the activation means are provided in the apparatusfor applying to the electrodes electric signals multiplexed in time.

In one embodiment, each of the components and/or systems of theapparatus is associated to an electronic component of transistor typeplaced on the spots of the matrix of the matrix arrangement.

Other features and advantages of the device according to the inventionwill become apparent upon reading the detailed description and examplesgiven below for illustrative purposes only.

BRIEF DESCRIPTIONS OF THE FIGURES

FIG. 1 shows a diagram of an embodiment of the device as described inthe invention

FIG. 2 shows the absorbance and fluorescence spectra of threepopulations of CdSe nanosheets emitting at 460, 512 and 550 nm.

FIG. 3A shows the radiation diagram of an anisotropic flat colloidalnanocrystal.

FIG. 3B shows the radiation pattern for an assembly of nanocrystals witha certain number of flat nanocrystals oriented in the same direction. Ifall nanocrystals have a random direction the radiation pattern isisotropic. On the other hand if all the nanocrystals are oriented on thesame direction, the pattern is the same as for a single nanocrystal theradiation diagram of an anisotropic flat colloidal nanocrystal

FIG. 3C shows a scheme of flat nanocrystals lying on a substrate with asmall tilt of their normal direction compared to the normal to thesubstrate.

FIG. 3D shows a scanning electron microscope image of CdTe nanoplateletson a graphene substrate, wherein all nanoplatelets lie on the samefacet.

FIG. 3E shows fluorescence intensity in false color for an assembly ofnanoplatelets oriented in a given direction (perpendicular to the needledirection) depending on the polarization direction. Scale bar is 10 nm.

FIG. 4 shows a sectional view of the schematic structure of alight-emitting diode containing anisotropic flat colloidal nanocrystals,as described in an embodiment of the invention.

FIGS. 5 a and 5 b show an example of a band structure of thesemiconductors composing a core/shell heterostructure of type I and typeII.

FIG. 6 shows sectional views of schematic structures of an example of acomponent as described in an embodiment of the invention.

FIGS. 7 a, 7 b and 7 c present sectional views of schematic structuresof supports containing flat nanocrystals as described in an embodimentof the invention.

FIGS. 8 a and 8 b are schematic views, respectively from above andsectional, of an embodiment of a component in which the normal to thenanocrystals is substantially parallel to the normal to the emittingface.

FIGS. 9 a and 9 b are schematic views, respectively from above andsectional, of an embodiment of a component in which the normal to thenanocrystals is substantially perpendicular to the normal to theemitting face.

FIG. 10 is a schematic diagram in cross section of an example of anorientation means of the sheets in regard to the emitting face.

FIG. 11 shows an embodiment of a device such as a display comprisingseveral components.

DETAILED DESCRIPTION

A first embodiment of a device for emitting light in response to theactivation of excitation means according to the invention is shown inFIG. 1. In the following, we denote by a flat nanocrystal an anisotropiccolloidal semiconductor nanocrystal whose smallest dimension, thethickness, is smaller than the other two by a factor of at least 1.5. Wedenote by sheet a flat nanocrystal having at least one dimension, thethickness, of nanometric size and large lateral dimensions compared tothe thickness, typically more than 5 times the thickness. We denote thenormal to the flat nanocrystal the normal to the largest flat surface ofthe nanocrystal.

This device comprises at least one flat nanocrystal 101, and anexcitation means 102 thereof. The light emitted by the device has anintensity and polarization that vary according to the angle formed bythe light emission direction 103 and the normal to the largest flatsurface of the nanocrystal 104. In one embodiment, the device comprisesat least one assembly of flat nanocrystals.

Flat nanocrystals are fluorophores whose emission wavelength can beselected with precision throughout the visible spectrum simply bychanging the composition and structure of said flat nanocrystals.

Dispersed in a transparent matrix, the flat nanocrystals are capable ofabsorbing any light radiation of wavelength less than their fluorescencewavelength and of re-emitting radiation at the fluorescence wavelength.These are thus wavelength converters. All the flat nanocrystals,whatever their composition and their fluorescence wavelength, have ahigh absorption cross section in the ultraviolet and blue. It is thuspossible to excite with a same ultraviolet or blue radiation differenttypes of flat nanocrystals that fluoresce in the blue, green and red forexample.

The fluorescence of a semiconductor nanocrystal comes from therecombination of an exciton in said nanocrystal. Given the nanometricsize of the nanocrystal, a quantum confinement effect is exerted on theexciton and shifts towards the blue the fluorescence wavelength withrespect to the exciton in the absence of confinement. The smaller thenanocrystal, the stronger the confinement effect that shifts towards theblue the fluorescence wavelength.

In the particular case of sheets, the thickness being much smaller thanthe lateral dimensions of the nanocrystal, the quantum confinementeffect is felt in the thickness only. In addition, in the sheets, thethickness can be well defined, at the atomic monolayer level. Thecombination of quantum confinement in one dimension only and perfectthickness control allows achieving the narrowest spectral fluorescenceever reported for isotropic semiconductor nanocrystals. Thus, asdescribed in patent WO2010/029508 an ensemble of colloidal sheets ofsemiconductor may have a very narrow fluorescence spectrum, the fullwidth at half maximum of the fluorescence peak being less than 12 nmExamples of absorbance and fluorescence spectra of solutions ofcolloidal sheets of semiconductors are shown in FIG. 2.

The inventors have found that the flat nanocrystals have a particularradiation pattern, shown in FIGS. 3A and 3B. A maximum is observed in apreferred direction, i.e. normal to the surface of the flat nanocrystal.

The inventors have also found that, in this preferred emissiondirection, the light emitted by the flat nanocrystal presents noparticular polarization but, conversely, the light emitted in the planeof the flat nanocrystal has a linear polarization following said flatnanocrystal plane.

Devices, components and apparatus as will be described in the followingexploit these two properties.

The excitation means of the device according to the invention can be ofseveral kinds.

They may comprise means of injecting electric charges, electrons and/orholes, in the flat nanocrystal as shown schematically in FIG. 4. Theelectrons and/or holes are injected into the flat nanocrystal 401through electrodes 402 and 403, of which at least one 403 is transparentin the spectral range of the light emitted by the nanocrystal and ofsupports 404 and 405, here semiconducting layers respectively of n-typeand p-type deposited on both sides of a layer of the nanocrystals 401.

This excitation means corresponds in particular to light-emittingdiodes. It may also correspond to the AC-TFEL devices (AlternatingCurrent Thin Film Electroluminescence) in which the charges aregenerated in an alternating of insulating and emissive films by theapplication of an alternating high voltage. The alternating electricfield induced by the applied voltage generates charges, especially atthe insulator/luminescent compounds interfaces.

The excitation means of the device may comprise means for applying tothe flat nanocrystal an electromagnetic field, or electromagnetic wavessuch as light waves selected such that at least part of said waves areabsorbed by the flat nanocrystal. The exciting means corresponds inparticular to wavelength converters containing fluorescent nanocrystals:they absorb some or all of the excitation light and emit light atanother wavelength, generally higher that the length of excitation.

The excitation means of the device may comprise at least one electric ormagnetic dipole coupled by electromagnetic interactions to the flatnanocrystal. This excitation means corresponds in particular to FRET(Forster Resonance Energy Transfer) type excitations. In this case, adonor fluorophore, initially in an excited electronic state can transferits energy to an acceptor fluorophore through a non-radiative dipolarcoupling. The donor fluorophore can be composed of organic fluorophores,fluorescent semiconductor nanocrystals or even quantum wells in anexcited state.

The flat nanocrystal can comprise group IV, group IIIA-VA, groupIIA-VIA, group IIIA-VIA, group IA-IIIA-VIA, group IIA-VA, group IVA-VIA,group VIB-VIA, group VB-VIA, or group IVB-VIA or/and group II-VI, III-V,IV-VI, II-V or I-III-VI semiconductors. In particular, it can contain atleast one of the following: Si, Ge, Sn, CdS, CdSe, CdTe, ZnS, ZnSe,ZnTe, HgS, HgSe, HgTe, PbS, PbSe, PbTe, CuInS₂, CuInSe₂, AgInS₂,AgInSe₂, CuS, Cu₂S, Ag₂S, Ag₂Se, Ag₂Te, InN, InP, InAs, InSb, In₂S₃,Cd₃P₂, Zn₃P₂, Cd₃As₂, Zn₃As₂, ZnO, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs,GaSb, FeS₂, TiO₂, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, MoS₂, WS₂, VO₂.

The flat nanocrystal can be doped with a lanthanide or a transitionmetal.

In one embodiment, the flat nanocrystal is covered with anothersemiconductor of different chemical composition. This type of structuresis generally named core/shell.

FIG. 5 a shows an example of the band structure of semiconductorscomposing a core 501/shell 502 type I heterostructure. When the energylevel of the conduction band of the semiconductor forming the shell 503is higher than the energy level of the conduction band of thesemiconductor forming the core 504 and the energy level of the valenceband of the semiconductor forming the shell 505, is lower than theenergy level of the valence band of the semiconductor forming the core506, the bandgap of the semiconductor composing the shell is of greaterwidth that the bandgap of the semiconductor composing the core, whichleads to the formation of a type I heterostructure, which confines theelectron and hole forming the exciton in the heart of theheterostructure.

FIG. 5 b shows an example of the band structure of semiconductorscomposing a core 501/shell 502 type II heterostructure. When the energylevel of the conduction band of the semiconductor forming the shell 503is lower than the energy level of the conduction band of thesemiconductor forming the core 504 and the energy level of the valenceband of the semiconductor forming the shell 505 is lower than the energylevel of the valence band of the semiconductor forming the core 506, theheterostructure is called type II. There is spatial separation of theelectron and the hole, the latter being located mainly in the core orthe shell.

The shell thickness may vary from one atomic monolayer to several tensof nanometers. Furthermore, it may be of uniform thickness throughoutthe whole nanocrystal, or conversely of heterogeneous thickness, beingthicker on the large faces of the anisotropic flat nanocrystal than onthe smaller faces, or vice versa.

The flat nanocrystal can be covered by organic molecules. Said organicmolecules can play the role of surface ligands, i.e. a function presenton the organic molecule may adsorb on the surface of the nanocrystal.The adsorption of these ligands changes the fluorescence properties ofthe anisotropic flat colloidal semiconductor nanocrystal; theiradsorption also provides colloidal stability of the nanocrystals. Theorganic molecules are selected from thiols, amines, carboxylic acids andphosphines.

In one embodiment of the invention, the flat nanocrystal comprises acolloidal semiconductor sheet. We denote as sheet a flat nanocrystalhaving at least one dimension, the thickness, of nanometric size andlarge lateral dimensions compared with the thickness, typically morethan 5 times the thickness. The flat nanocrystal can be itself a sheet.The flat nanocrystal can alternatively comprise a sheet in a core/shellstructure, for example.

In a second embodiment of the invention, a component is formed as shownin FIG. 6, comprising at least one device, itself comprising at leastone flat nanocrystal 601, and a means of excitation 602 thereof. This atleast one device is placed in a support 603, said support beingtransparent to the light emitted by the device in at least one direction604 towards an observer 605 located outside of the component, thesupport surface through which light is emitted towards the observer isdefined as emitting face 606.

The excitation means of the device may be integrated to the support oralternatively, the excitation means of the at least one device may beexternal to the substrate.

The component may comprise one or more devices and therefore one orseveral nanocrystals. In addition, in a component which comprises atleast two devices, the means of excitation of said devices may be sharedor not.

The component comprises a support in which is placed the at least onedevice, said support may be formed in various ways. It can for examplebe made of an organic material, for example a polymer or an inorganicmaterial; it can be made, as shown schematically in FIG. 7 a of one solematerial 702 in which the nanocrystals 701 are dispersed.

It may also comprise several different materials forming several layers.

In FIG. 7 b, the nanocrystals are located within one of the layers and,in FIG. 7 c; they are located at the interface between two differentlayers.

The support is transparent in at least one direction for the lightemitted from the nanocrystals, which implies that some of the layers 703may be opaque or reflective to the light emitted from the nanocrystals,while another portion 704 must be transparent therefor. Depending on theexcitation means used, the support can also be transparent with respectto excitation light of the nanocrystals. Finally, following other meansof excitation used, the support may comprise electrodes as well asorganic or inorganic semiconducting layers.

The support may be made in various ways: it may contain a liquid,particularly a liquid with properties of liquid crystals.

It may also comprise at least one organic material such as a well-knownmaterial for the production of light-emitting diodes with smallmolecules such as Alq3.

The support may contain at least one polymer material:

-   -   The polymer may comprise chemical functions capable of        substituting themselves to the surface ligands of the        nanocrystals.    -   The polymer may be a polyelectrolyte.    -   The polymer may be a poly(methyl methacrylate), a polystyrene, a        polycarbonate, a polyethylene, a polyethylene terephtalate, an        epoxide, a polyester, a polysiloxane.    -   The polymer may be a semiconducting polymer such as a        polythiophene, P3HT, MDMO PPV, MEH-PPV, PEDOT, PEDOT:PSS, PCBM,        PCNEPV, polyfluorene, PSS.

The support may comprise an inorganic material:

-   -   The inorganic material may be a semiconductor such as a II-VI        semiconductor: CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, III-V: AlN,        GaN, InN, AlP, GaP, InP, AlAs, GaAs, InAs and alloys thereof, an        intrinsic semiconductor such as carbon, silicium or germanium.    -   The semiconductor may be doped.    -   The inorganic material may be a glass such as silica.    -   The inorganic material may be an oxide: TiO2, ITO (indium oxide        doped with tin), NiO, ZnO, SnO2, SiO2, ZrO2.    -   The inorganic material may be a metal: gold, silver, molybdenum,        aluminum . . . .

In one embodiment of the invention, a component may comprise at leasttwo devices, the at least two anisotropic flat colloidal semiconductornanocrystals of the least two devices having different characteristics.In one embodiment of the invention, a component may comprise at leasttwo devices, the at least two assemblies of anisotropic flat colloidalsemiconductor nanocrystals of the least two devices having differentcharacteristics. Said characteristics may be the dimensions or chemicalcompositions. This allows defining a component featuring two distinctpopulations of nanocrystals that emit at two different wavelengths.

The peculiar emission diagram of the flat colloidal semiconductor can beutilized if the nanocrystals of the component are oriented. It ispossible to define their orientation either relative to the othernanocrystals present in the component or relative to the emitting faceof the component. For this it is necessary to introduce a normal to theanisotropic flat colloidal semiconductor nanocrystal, which will fullydefine the orientation of the nanocrystals. The normal to thenanocrystal is defined as the normal to the largest surface of the flatnanocrystal.

If one orients the nanocrystals relative to the emitting face, one candefine a component wherein the normal to at least one nanocrystal issubstantially parallel to the normal to the emitting face, as shown inFIG. 8 a as seen from above and in cross section FIG. 8 b. If the flatnanocrystals 801 are all oriented in the component so that the normalsto the flat nanocrystals 802 are all aligned and parallel to the normal803 to the emitting face 804, one will have en emitter emittingpredominantly in the direction of the incident beam, but the reemittedradiation will not be polarized. In one embodiment, more than 50%, 55%,60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of the anisotropic flatcolloidal semiconductor nanocrystals of the assembly have their normal802 substantially parallel to the normal of the emitting face 804.

Similarly, if one orients the nanocrystals relative to the emittingface, one can define wherein the normal to at least one nanocrystal issubstantially perpendicular to the normal to the emitting face, as shownin FIG. 9 a as seen from above and in cross section FIG. 9 b. If theflat nanocrystals 901 are all oriented within the component so that thenormals to the flat nanocrystals 902 are all aligned and perpendicularto the normal 903 to the emission surface 904, one will have an emitteremitting a linearly polarized radiation. In one embodiment, more than50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of the anisotropicflat colloidal semiconductor nanocrystals of the assembly have theirnormal 902 substantially perpendicular to the normal of the emissionsurface 904.

Intermediate orientation situations give rise to components reemitting alight partially polarized and/or with a less pronounced directivity.

To orient the nanocrystals with respect to each other, the componentmust contain at least two nanocrystals, the normals to the surfaces ofthe at least two nanocrystals being substantially parallel to a givendirection. In particular, substantially all of the component'snanocrystals have their normal to their surface substantially parallelto a given direction. This case is shown in particular in FIGS. 8 and 9,wherein all of the nanocrystals of the component have the normals totheir surfaces substantially parallel to a given direction, saiddirection being respectively either the normal to the emission surfaceof the component or a perpendicular to the emission surface of thecomponent. In one embodiment, more than 50%, 55%, 60%, 65%, 70%, 75%,80%, 85%, 90%, or 95% of the anisotropic flat colloidal semiconductornanocrystals of the assembly of the component have the normal to theirsurface substantially perpendicular or substantially parallel to a givendirection, said direction being preferably either the normal to theemission surface of the component or a perpendicular to the emissionsurface of the component.

To orient the nanocrystals within the component, the support maycomprise means for orienting them according to at least one preferreddirection.

Due to the flat shape of the nanocrystals, they can deposit flatly onsurfaces. A first means for orientating flat nanocrystals is presentedin FIG. 10 is to deposit them on a surface 1002 of given orientationlocated in the support, the nanocrystals 1001 will deposit flatly onsaid surface, the normal of the nanocrystals 1003 will thus be parallelto the normal to the surface. The methods of manufacturing a componentcomprising oriented nanocrystals may involve a support surface withinthe component on which the nanocrystals are deposited.

It is possible to implement a process for manufacturing a component inwhich the nanocrystals are deposited on a support surface with thenormal to their surface substantially parallel to the normal to thesupport surface.

It is possible to implement a process for manufacturing a component inwhich the support surface comprises a texturing. Indeed, the supportsurface may not be flat but microstructured for example. A blazedgrating for example, would allow to predominantly orient thenanocrystals according to an angle corresponding to the angle of theblazed grating.

To make a component integrating oriented nanocrystals, it can beadvantageous to deposit the nanocrystals on a substrate by a method thatallows flat nanocrystals to settle flatly on the substrate. The processof manufacturing the component may comprise:

-   -   a step of depositing the nanocrystals by slow evaporation on the        substrate of a solution containing them,    -   a step of depositing the nanocrystals by spin coating,    -   a step of depositing the nanocrystals by dip coating    -   a step of depositing the nanocrystals through layer-by-layer        electrostatic adsorption.

Alternatively, a film of oriented nanocrystals can be preparedbeforehand and then transferred onto a substrate. In this case, anddepending on the mode of preparation of the film, it may comprisenanocrystals either predominantly horizontal or predominantly verticalrelative to the film itself. It is possible to implement a process ofmanufacturing the component comprising a step of depositing thenanocrystals by formation and transfer of a Langmuir-Blodgett orLangmuir-Schaefer film.

Finally, it may be useful to deposit on the flat nanocrystals aprotective layer, a glass or a polymer, for example. Said process beingrepeatable in order to form multilayers “matrix/flat nanocrystals”composites.

An alternative to orient the nanocrystals is to prepare beforehand asolution containing the flat nanocrystals and a polymer or a polymerprecursor then polymerize and/or evaporate the solvent in the presenceof an electric field.

Another alternative to orient the nanocrystals is to prepare beforehanda thermoformable polymer/flat nanocrystal composite, the flatnanocrystals will orient in the matrix due to stress applied during theshaping of the composite. In a third embodiment, an apparatus isrealized as shown in FIG. 11, comprising at least one component 1101-ias described above, each component may be subjected to activation means1102-i independently one another. Such a device can be a display forpresenting to an observer images, text or other visual information. Theactivation of the excitation means of the devices forming the apparatusis typically done using an electric current. To this end, an apparatusin which the means of activation are of electric type may comprise atleast two networks of electrodes in a matrix arrangement. Moreover,means can be provided for applying to the electrodes electric signalsmultiplexed in time. The apparatus can also be constructed so that eachcomponent is associated with a transistor type electronic componentplaced at spots of the matrix in the matrix arrangement.

This addressing can be done in any known manner. For example, in thecase of a LED display, it is the diodes themselves that are addressedmatrix-wise. In the case of a display with a common lighting to allpixels, such as a liquid-crystal display device, it is the shutters thatare addressed matrix-wise.

Due to the narrow full width at half maximum of the fluorescence peak ofanisotropic flat semiconductor nanocrystals in general and sheets inparticular, the device may have a very narrow emission spectrum width afull width at half maximum of the emission peak lower than 20 nm

EXAMPLES Example 1 Synthesis of Flat Core/Shell Fluorescent Nanocrystals

We describe for example, the synthesis of flat semiconductor fluorescentnanocrystals with a core/shell structure.

Synthesis of Sheets Emitting at 510 Nm:

The CdSe sheets for example can be obtained by any know method, such asdescribed for example in the following documents: Ithurria, S.;Dubertret, B. Journal of the American Chemical Society 2008, 130,16504-5 and Ithurria, S.; Bousquet, G; Dubertret, B. Journal of theAmerican Chemical Society 2011, 133, 3070-7. 174 mg of Cd(myristate)₂and 12 mg of Selenium powder are introduced in a three-necked-flaskcontaining 16 mL of 1-octadecene (ODE, 90%). The flask is degased underreduced pressure and under stirring at room temperature for 30 minutes.Argon is introduced in the flask which is then heated. When thetemperature reaches 200° C., 40 mg of Cd(Acetate)₂(H₂O)₂ are swiftlyintroduced in the reaction medium inducing the growth of the sheets. Thereaction medium is heated at 240° C., temperature at which it ismaintained for approximately 10 minutes in order to allow the growth ofthe sheets. The solution is then cooled down and washed by successiveprecipitations and suspensions. The first precipitation is done byaddition of oleic acid (approx. 10 mL), a non-solvent: ethanol (approx.70 mL) and centrifugation (5000 rpm for 10 minutes). The supernatant isdiscarded and the precipitate is suspended in hexane (approx. 10 mL).The solution containing the sheets still contains a large quantity ofCd(myristate)₂ used in excess. Addition of a few milliliters ofoctylamine (4 mL) allows its dissolution. The suspension of platelets isthen precipitated a second time by addition of ethanol (80 mL) andcentrifugation. The suspension/precipitation process is repeated againtwice with octylamine and a last time simply by suspension in hexane andprecipitation with ethanol. The final precipitate is suspended in 10 mLof hexane.

Treatment Aiming at Increasing the Fluorescence Quantum Yield:

After the successive washing steps, the synthesized sheets exhibit aweak fluorescence (quantum yield less than 1%). It is possible to havethem recover a high quantum yield (several tens of percent) by amodification of the surface ligands. To the solution of sheets in hexaneis added 200 μL of oleic acid and 20 mg of Cd(Acetate)₂(H₂O)₂. Thesolution is then heated at reflux for 2 hours.

Growth of a Shell on the Initial Sheets:

We describe for example the deposition of a film of Cd_(0.7)Zn_(0.3)S onthe initial CdSe sheets.

In a vial are introduced successively 2 mL of chloroform, 400 μL of theCdSe sheets solution, 20 mg of thioacetamide (TAA) and 200 μL ofoctylamine. The solution is placed under magnetic stirring for one hour,which causes the complete dissolution of the TAA then a color change ofthe solution. 4 mg of CdCl₂ and 3 mg of Zn(NO₃)₂ are then introducedinto the sheets solution. Said solution again gradually changes colorwhile the fluorescence quantum yield strongly increases. The precursorsare left reacting on the sheets for 24 hours. The sheets are thenaggregated by addition of a few drops of ethanol and the solution iscentrifuged 5 minutes at 5000 rpm. The supernatant containing theprecursors that have not reacted as well as the CdZnS parasitenanocrystals is discarded and the pellet formed by the sheets isdispersed in 2 mL of chloroform to which 20 mg of a solution ofCd(tetradecylphosphonate)₂ (Cd(TDPA)₂) 0.5 M in 1-octadecene are added.The fluorescence quantum yield then drops drastically. It increasesafterwards under ultraviolet lighting. The sheets are then purified byprecipitation with ethanol, centrifugation and suspension in hexane.This purification step can be carried out several times.

Alternatively, the CdSe/CdZnS sheets can be rendered dispersible in apolar medium. For this, the deposition step of Cd(TDPA)₂ is replaced bya deposition step of cadmium di-mercaptopropionate (Cd(MPA)₂). Once theCdSe/CdZnS sheets are separated by centrifugation, they are dispersed in2 mL of an aqueous solution of Cd(MPA)₂ at 0.1 M and pH 11. After 10minutes of sonication, the mixture is centrifuged the supernatantdiscarded and 2 mL of distilled water are added on the precipitate. Thesheets are then perfectly dispersible in aqueous solution.

Alternatively, in a vial are successively introduced 4 mL of chloroform,1 mL of the CdSe sheets solution, 100 mg of thioacetamide (TAA) and 1 mLof octylamine. The solution is submitted to sonication for 5 minuteswhich causes the complete dissolution of TAA and a color change of thesolution from yellow to orange. 350 μL of a 0.2 M solution of Cd(NO₃)₂in ethanol and 150 μL of a 0.2 M solution of Zn(NO₃)₂ in ethanol arerapidly injected in the sheets' solution. It gradually changes colorswhile the quantum yield greatly increases. The precursors are leftreacting for another 24 h at room temperature. The sheets are thenaggregated by addition of a few drops of ethanol and the solution iscentrifuged for 5 minutes at 4000 rpms. The supernatant containing theunreacted precursors as well as parasite CdZnS nanocrystals is discardedand the pellet containing the sheets is dispersed in 5 mL of chloroform.In order to increase the stability and the quantum yield of the sheets,100 μL of a 0.2 M solution of Zn(NO₃)₂ in ethanol are added to thesolution of platelets. They instantly aggregate and are dispersed byaddition of 200 μL of oleic acid.

Example 2 Deposition of Flat Semiconductor Nanocrystals on a PlanarSubstrate, the Normal to the Sheets Parallel to the Normal to theSurface of the Planar Substrate

We describe for example the deposition of flat semiconductornanocrystals on a planar substrate, so as the normal to the flatsemiconductor nanocrystals is parallel to the normal to the surface ofthe planar substrate according to a particular embodiment of theinvention.

The sheets can deposit flatly on a surface when they are deposited byevaporation of a diluted solution containing them. It is possible to useseveral techniques taking advantage of this property.

A first possibility is to deposit the sheets by direct evaporation of asmall amount of solvent containing the sheets. 1 mg of sheets isdispersed in 2 mL of a 9:1 in volume mixture of hexane and octane. Onedrop of the mixture is deposited on a glass slide. Due to the presenceof octane, the drop spreads and dries evenly without forming ring withhigh concentration of sheets at the edges (“coffee ring effect”). Oncethe drop has dried, the deposited sheets are homogeneously distributedover the entire surface of the stain and they exhibit predominantly anorientation in which the normal of the nanocrystals is parallel to thenormal to the surface.

A second possibility is to deposit the sheets by spin coating. At first,a microscope glass slide (26 mm by 26 mm) is cleaned with an oxygenplasma. It is then functionalized with 3-mercaptopropyl-triethoxysilaneby immersion for 10 minutes in a 1% solution in volume of3-mercaptopropyl-triethoxysilane in ethanol. The glass slide is rinsedthree times with ethanol and dried. A 1 mg/mL solution of sheets inhexane is then deposited on the slide by spin coating at 1000 rpm for 30seconds.

A third possibility is to deposit the sheets by dip coating. A glassslide previously washed with isopropanol is immersed in a 1 mg/mLsolution of the sheets in hexane. The glass slide, oriented such as thenormal to its surface is in the same plane as the air/solutioninterface, is slowly extracted from the solution at a constant speed of1 cm by minute, so as to form an homogeneous film of sheets on the glasssurface.

A fourth possibility is to deposit the sheets by depositing aLangmuir-Blodgett or Langmuir-Schaefer film of nanocrystals on thesubstrate. For this, a Langmuir film of nanocrystals is made by leavinga hexane solution of nanocrystals slowly evaporate on a surface ofdiethylene glycol (DEG) contained in a Teflon vessel. The resulting filmcan then be condensed with a Teflon bar dividing the surface of the tankinto two parts. Once the film has reached the desired density, it isremoved, either by applying directly the coating surface (deposition ofLangmuir-Schaefer) or by slowly extracting the target surface from theDEG while maintaining the desired density for the film by decreasing itssurface using the Teflon bar.

Example 3 Deposition of Sheets on a Microstructured Substrate

The microstructured substrate is a blazed grating. Neither directevaporation nor spin coating can be applied here. The deposit by soakingallows on the contrary to obtain a homogeneous film of sheets on thesurfaces of the blazed grating. The protocol described in the previousexample is applied here, it is only necessary to ensure that the gratingis extracted from the solution with the lines that compose itperpendicular to the liquid surface.

Example 4 Fabrication of a Light-Emitting Diode Using SemiconductingPolymers

We describe for example the fabrication of a light-emitting diode usingsemiconductor polymers according to a particular embodiment of theinvention.

The light emitting diode is generally comprises a first electrode, ahole conducting polymer layer, a layer of semiconductor nanocrystals, alayer of electron conducting polymer and a second electrode such asshown schematically in FIG. 4. The electron or hole conducting layersare optional if the Fermi level of the electrode enables the directinjection of charges. It is also possible to insert an electron blockinglayer or a hole blocking layer.

The glass substrate coated with an ITO transparent electrode(commercially available) is first washed with isopropanol and piranhamixture. On the clean substrate is then deposited by spin coating alayer of poly(3,4-ethylendioxythiophene):poly(styrenesulfonate)(PEDOT:PSS hole conducting layer) 30 nm thick. The assembly is thenannealed at 250° C. for 20 minutes. The emissive layer consisting ofCdSe/CdZnS sheets is then made by spin coating at 2500 rpm of a 10 mg/mLsolution of nanocrystals in octane. Finally, the counter-electrode ismade by thermal evaporation of 2 nm LiF (lithium fluoride) and 100 nmaluminum.

Example 5 Fabrication of a Light-Emitting Diode Using SemiconductorOxides

We describe for example, the fabrication of a light-emitting diode usingsemiconductor oxides according to one embodiment of the invention.

The light-emitting diode comprises a first transparent ITO (indium tinoxide) electrode, a layer of nickel oxide, a layer of fluorescentcolloidal nanocrystals, a zinc oxide layer and a silver electrode, asshown schematically in FIG. 4.

On a clean glass substrate, an ITO anode 60 nm thick is deposited bymagnetron sputtering through a mask in an inert environment of argon ata pressure of 5 mTorr and a speed of 0.06 Å·s⁻¹. The substrate is heatedat 250° C. during the deposition to increase the conductivity of theITO. A 20 nm thick layer of p-doped NiO (hole conducting layer) is thendeposited also by magnetron sputtering at a speed of 0.2 Å·s⁻¹ in a2:100 oxygen in argon atmosphere and a pressure of 6 mTorr.

A dense layer of CdSe/CdZnS sheets is deposited on the NiO by spincoating of a solution of sheets dispersed in hexane under nitrogen. Theconcentration of the sheets solution is adjusted so as to obtain adeposit of about ten nanometers thick.

The electron conducting layer is then deposited on the sheets bysimultaneous cathodic deposition of ZnO at 15 W and SnO₂ at 9 W underargon at a pressure of 5 mTorr. The deposition rate is 0.2 Å·s⁻¹.

Finally, the 40 mm thick silver electrode is deposited by thermalevaporation through a mask on the ZnO:SO₂ layer at a rate of 1.5 Å·s⁻¹.

Example 6 Fabrication of a Polymer Film Containing the FlatSemiconductor Nanocrystals-Wavelength Converter

We describe for example, the fabrication of a polymer film containingthe flat semiconductor nanocrystals-wavelength converter according toone embodiment of the invention.

The polymer used is a statistic polymer containing 95% methylmethacrylate and 5% acrylic acid. It is a commercially availablepoly(methyl methacrylate)-polyacrylic acid (PAA-PMMA).

In a three-necked flask are introduced the toluene solution of sheets,10 mg of Cd(Acetate)₂(H₂O)₂ and 2 g of polymer (PMMA-PAA) previouslydissolved in 10 mL of anisole. The mixture is heated at 100° C. for 2hours with magnetic stirring. After cooling to room temperature, themixture has a high quantum yield. The polymer/sheets composite isprecipitated by adding 10 mL of hexane and then centrifuged. Thesupernatant is discarded and the precipitate dissolved in a fewmilliliters of anisole. The resulting composite can be shaped.

In one embodiment, the shaping of the composite is done by spin coatingto form a thin film of polymer/sheets composite. In another embodiment,the composite is dried and is shaped by thermoforming

Example 7 Fabrication of a Nanocrystals/Polymer Composite Film ThroughLayer-by-Layer Electrostatic Assembly

In one embodiment of the invention, a rigid substrate (glass slide orPMMA slide for example) or a flexible substrate (polyethylene film oflow density for example) previously cleaned is used as support. Apolycationic polymer film, poly(diallyldimethylammonium chloride) (PDDA,Mw=5000-20000) is deposited on the support by dipping it into a 20 g/Lsolution containing PDDA at pH 9 (adjusted by addition of TBAOH:tetrabutylammonium hydroxide) for 20 minutes. After rinsing withultrapure water (>18MΩ cm) the support is immersed in a 100 mg/L aqueoussolution of sheets stabilized with mercaptopropionic acid (negativelycharged) for 20 minutes as well. The film formed is rinsed withultrapure water.

In one embodiment of the invention, a multilayer film can be obtained byrepeating the two adsorption steps of PDDA and the sheets.

Example 8 Fabrication of a Polymer Film Comprising Sheets as Well asFluorescent Nanocrystals. Excitation by Non-Radiative Energy Transfer

The excitation by non-radiative transfer of energy is carried by FRET(Forster Resonance Energy Transfer) between an acceptor fluorophore,which is a core/shell CdSe/CdZnS water-soluble sheet prepared accordingto example 1 and donor fluorophores: ZnSe/ZnS colloidal semiconductornanocrystals prepared by any known method.

In order to implement the excitation by FRET of the semiconductorsheets, the ZnSe/ZnS nanocrystals must be located as close as possibleto the semiconductor layers. At first, one will prepare a dispersion ofsheets on which are adsorbed the ZnSe/ZnS nanocrystals.

For this, the ZnSe/ZnS nanocrystals are first exchanged with a cationicwater-soluble ligand according to any known method, the water-solublecationic ligand may, in particular, be a dihydrolipoic acidfunctionalized with an amine or a quaternary ammonium.

10 mg of water-soluble CdSe/CdZnS sheets having a negative surfacecharge are dispersed in 10 mL of ultrapure water. To this solution isadded dropwise 10 mL of a solution of 40 mg of previously synthesizedZnSe/ZnS nanocrystals bearing a positive surface charge dispersed inwater. Sheets and nanocrystals assemble by electrostatic interactions.Sheets/nanocrystals complexes are purified by centrifugation anddispersed in 10 mL of ultrapure water. To this solution is added a 10 mLof a 10 mg/mL aqueous solution of PVA (polyvinyl alcohol). The solutionis placed in a mold of appropriate size and the water is evaporated inan oven at 65° C. until complete drying of the composite film.

Example 9 Use of Flat Semiconductor Nanocrystals in the Fabrication of aBacklit Screen

In one embodiment of the invention, the semiconductor nanocrystals aredeposited on a planar transparent substrate that can be flexible likeplastic or rigid like glass. The substrate is placed between the backlitsource emitting in the blue and the external transparent surface of thescreen during its manufacture.

Example 10 Use of Flat Semiconductor Nanocrystals in the Fabrication ofan Emissive Display

In one embodiment of the invention, the flat semiconductor nanocrystalsare deposited on a planar substrate to form a dense assembly that can beoriented. Flat semiconductor nanocrystals are transferred using a pad ona matrix designed to be able to excite the nanocrystals so that theyemit light.

Example 11 Use of Electrophoretic Method to Deposit the Nanocrystal

The nanocrystals are initially dispersed in a non-polar solvent such ashexane. Two ITO (indium tin oxide) coated glass substrate are used andspaced by 1 cm. A high DC bias source is used to apply a 1 kV biasbetween the two ITO coated substrate. The deposition is conducted for 1min to 1 h depending on the nanocrystal surface chemistry. Depositiononly occurs on the positive electrodes. After the deposition the film isquickly rinsed on fresh acetone to clean the surface and get rid of theexcess of ligand.

BIBLIOGRAPHY

-   1. Amstutz et al. U.S. Pat. No. 4,634,229-   2. Shi et al. U.S. Pat. No. 5,705,285-   3. Suzuki et al. worldwide patent application No. WO2010/028728-   4. Jeong et al. Korean patent application No. 1020100089606-   5. Kazlas, P.; Linton, J. R. worldwide patent application No.    WO2009/011922-   6. Rho et al. Korean patent application No. 1020090036373-   7. Sokolik, I.; Campos, R. A. worldwide patent application No.    00/17903-   8. Bawendi et al. U.S. Pat. No. 6,501,091-   9. Kim, et al. Nature Photonics 2011, 5, 176-182.-   10. Abécassis, et al. Nano Lett 2014, 14, 710.-   11. Cassette et al. ACS Nano, 2012, 6, 6741.

1. A component to emit light in response to the activation of excitationmeans, comprising a support and a plurality of anisotropic flatcolloidal semiconductor nanocrystals whose smallest dimension, namelythe thickness, is smaller than the other two dimensions by a factor ofat least 1.5, wherein at least 50% of the plurality of nanocrystals havetheir normal to their largest surface substantially parallel to a givendirection.
 2. The component according to claim 1, wherein at least 50%of the plurality of anisotropic flat colloidal semiconductornanocrystals have their normal to the largest surface substantiallyparallel or substantially perpendicular to the support.
 3. The componentaccording to claim 1, wherein the plurality of anisotropic flatcolloidal semiconductor nanocrystals is a plurality of anisotropic flatcolloidal semiconductor nanosheets.
 4. The component according to claim1, wherein the plurality of anisotropic flat colloidal semiconductornanocrystals comprises at least one compound of group IV, group III-V,group II-VI, group III-VI, group I-III-VI, group II-V or group IV-VI, ormixture thereof.
 5. The component according to claim 1, wherein theplurality of anisotropic flat colloidal semiconductor nanocrystalscomprises at least one of the following compounds: Si, Ge, CdS, CdSe,CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, PbS, PbSe, PbTe, CuInS₂,CuInSe₂, AgInS₂, AgInSe₂, CuS, Cu₂S, Ag₂S, Ag₂Se, Ag₂Te, InN, InP, InAs,InSb, In₂S₃, Cd₃P₂, Zn₃P₂, Cd₃As₂, Zn₃As₂, ZnO, AlN, AlP, AlAs, AlSb,GaN, GaP, GaAs, GaSb FeS₂, TiO₂, Bi₂S₃, Bi₂Se₃, Bi₂Te₃.
 6. The componentaccording to claim 1, wherein the plurality of anisotropic flatcolloidal semiconductor nanocrystals comprises at least oneheterostructure comprising an anisotropic flat colloidal semiconductornanocrystal.
 7. The component according to claim 1, wherein theplurality of anisotropic flat colloidal semiconductor nanocrystalscomprises at least one anisotropic flat colloidal semiconductornanocrystal totally covered with a semiconductor of different chemicalcomposition.
 8. The component according to claim 1, wherein theplurality of anisotropic flat colloidal semiconductor nanocrystalsexhibits a narrow fluorescence spectrum, with a full width at halfmaximum less than 100 nm, 75 nm, 50 nm, 40 nm, 30 nm, 25 nm, 24 nm, 23nm, 22 nm, 21 nm or 20 nm.
 9. The component according to claim 1,wherein the support is transparent for the emitted light in at least onedirection towards an observer located outside the component, the supportsurface through which the light is emitted towards the observer beingdefined as emitting face.
 10. The component according to claim 1,wherein the support comprises a liquid having liquid crystalsproperties.
 11. The component according to claim 1, wherein the supportis flexible or rigid.
 12. The component according to claim 1, whereinthe support comprises an inorganic material or an organic material. 13.The component according to claim 1, wherein the support comprises apolymer material.
 14. The component according to claim 1, wherein theplurality of anisotropic flat colloidal semiconductor nanocrystalscomprises at least two anisotropic flat colloidal semiconductornanocrystals having different characteristics, dimensions and/orchemical compositions and/or emission wavelengths.
 15. The componentaccording to claim 1, wherein the emitted light has an intensity and apolarization which vary according to the angle formed by the lightemitting direction and the normal to the largest surface of theplurality of anisotropic flat colloidal semiconductor nanocrystals. 16.A light emitting system comprising a component according to claim 1 andat least one excitation means comprising means for applying to theplurality of anisotropic flat colloidal semiconductor nanocrystals anelectromagnetic field, in particular a light source, wherein at least aportion of the emitted light is absorbed by the plurality of anisotropicflat colloidal semiconductor nanocrystals, such as a gallium nitridediode.
 17. An apparatus comprising at least one system according toclaim 16.